How to add sound to sequencer using python
import bpy
from bpy import context

scene = context.scene 

if not scene.sequence_editor:
    scene.sequence_editor_create()

#Sequences.new_sound(name, filepath, channel, frame_start)    
soundstrip = scene.sequence_editor.sequences.new_sound("toad", "~/toad_sound.mp3", 3, 1)